The use of Nb superconducting accelerator tubes in particle accelerators is well known in the art. A great deal of effort has been devoted to improving the manufacturing techniques used to produce such tubes because of their generally very high cost as dictated by the very stringent internal surface quality requirements which must be met to achieve optimum accelerator operating efficiency.
Superconducting RF accelerator cavities are commonly produced from preformed Nb half-cells joined together by welding. Currently, electron beam welding or EBW is the process of choice for this assembly step.
EBW is a violent process leaving a highly disturbed weld puddle with occasional voids, defects and a irregular surface texture. A surface irregularity can result in local magnetic field enhancement. If this enhancement causes the field to exceed a critical level, a so-called "normal" zone or region may occur, wherein the Nb undergoes a transition from the superconducting state to the normal conducting state. The propagation of such a "normal" zone is limited by the thermal conductivity (RRR) of the surrounding Nb. If the propagation of a "normal" zone is not stopped, the entire cavity may quench or become "normal" over all, or a substantial portion, of its surface. In this condition, the maximum accelerating field at which the cavity can operate will be significantly limited. These problems may be further compounded by the fact that the thermal conductivity of a solidified weld bead is often significantly reduced by the vacuum levels used in a typical electron beam welder.
In addition to the above potential problems which may be caused by weld defects, the EBW process tends to be very expensive primarily due to the high cost of precision machining of all faying surfaces to avoid the occurrence of weld defects. Previously used weld parameter sets have required a carefully machined edge in order to achieve a satisfactory weld. For example, in welding a cavity from 3 mm niobium, machined overlaps 1.5 mm thick were provided as the faying surfaces. Such edge preparation is extremely time consuming and expensive.
An ideal electron beam weld and welding process would therefore have the following properties:
1) It would provide reduced fabrication costs by having a high tolerance for variations in faying surface preparation, i.e. sheared edges having cracks, burrs and other irregularities could be used without further precision machining; PA1 2) It would provide a very smooth weld surface on the inside of the cavity when welded from the outside in the presence of such irregularities; and PA1 3) The process would raise rather than degrade the thermal conductivity of the weld bead and the surrounding heat affected zone. PA1 Beam Voltage: 45 KV to 55 KV PA1 Beam Current: 38 ma to 47 ma PA1 Weld speed: 15 cm/min PA1 Focus: Sharp PA1 Rhombic Raster: 9 KHz and 10 KHz axes.